Address feedback from code review.

* Switch "an SAS" back to "a SAS"
* Remove the `next_method` field from m.key.verification.start$m.sas.v1
  but add additional clarification to its description on
  m.key.verification.start that it is never present for methods that
  verify keys both ways.

event-schemas/schema/m.key.verification.start

@@ -28,7 +28,8 @@ properties:
         type: string
         description: |-
           Optional method to use to verify the other user's key with. Applicable
-          when the ``method`` chosen only verifies one user's key.
+          when the ``method`` chosen only verifies one user's key. This field will
+          never be present if the ``method`` verifies keys both ways.
     required:
       - from_device
       - transaction_id

event-schemas/schema/m.key.verification.start$m.sas.v1

@@ -3,7 +3,7 @@ allOf:
   - $ref: core-event-schema/event.yaml
 
 description: |-
-  Begins an SAS key verification process using the ``m.sas.v1`` method. Typically sent as a `to-device`_ event.
+  Begins a SAS key verification process using the ``m.sas.v1`` method. Typically sent as a `to-device`_ event.
 properties:
   content:
     properties:
@@ -23,10 +23,6 @@ properties:
         enum: ["m.sas.v1"]
         description: |-
           The verification method to use.
-      next_method:
-        type: string
-        description: |-
-          Optional method to use to verify the other user's key with.
       key_agreement_protocols:
         type: array
         description: |-